Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 66. Carbaboranetungsten–platinum complexes. Polyhedral rearrangements of a 12-vertex cage system; crystal structures of [PtW(CO)2(PEt3)2{µ6-C2B9H8(CH2C6H4Me-4)Me2}]·CH2Cl2, [PtW(µ-H){µ-σ:η5- C2B9H7(CH2C6H4-Me-4)Me2}(CO)2(PMe3)(PEt3)2], and related compounds

Abstract

In acetone, the salts [N(PPh3)2][W(CC6H4Me-4)(CO)2(η5-C2B9H9Me2)] and [PtH(Me2CO)(PEt3)2][BF4] yield the dimetal compound [PtW(CO)2(PEt3)2{η6-C2B9H8(CH2C6H4Me-4)Me2}](2a), the structure of which has been established by X-ray diffraction. The very short Pt–W bond [2.602(1)A] is semi-bridged by the two carbonyl ligands [W–C–O 165(1) and 168(1)°], while a novel η6-C2B9H8(CH2C6H4Me-4)Me2 group ligates the tungsten atom. Six atoms [graphic omitted] in the face of the ligand are within bonding distance of the metal, but the C ⋯ C separation (2.88 A) within the ring is non-bonding. The CH2C6H4Me-4 substituent is attached to the central boron of the B3 unit, and is derived from the Pt–H, B–H, and CC6H4Me-4 groups present in the reactants. Treatment of [N(PPh3)2][W(CMe)(CO)2(η5-C2B9H9Me2)] with [ PtH (Me2CO)(PEt3)2][BF4] affords an inseparable 1 : 2 mixture of the two complexes [PtW(CO)2(PEt3)2{η6-C2B9H8(R)Me2}][(2b), R = H; (2c), R = Et]. Reactions between (2a) and the electron pair donor molecules L = PMe3, CO, CNBut, or PHPh2 yield the complexes [PtW(µ-H){µ-σ:η5-C2B9H7(CH2C6H4Me-4)Me2}(CO)2(L)(PEt3)2](3). The structure of (3a)(L = PMe3) was established by X-ray diffraction, confirming that the Pt–W bond [2.843(2)A] is bridged by a hydrido ligand, the presence of which was revealed in the 1H n.m.r. spectrum. The tungsten atom is ligated by two terminal CO groups, the PMe3 group, and an η5-C2B9H7(CH2C6H4Me-4)Me2 group. In the latter the central boron of the nido-C2B3 face carries the CH2C6H4Me-4 substituent, while another boron atom in the face forms an exo- polyhedral bond to the platinum [2.123(5)A]. The synthesis of (3a) from (2a) thus represents an unprecedented iso-closo(or hyper-closo) to closo transformation in a C2B9W 12-vertex framework. The mixture of compounds (2b) and (2c)(ca. 1 : 2) reacts with CO to give three complexes [PtW(µ-H){µ-σ:η5-C2B9H7(R)Me2}(CO)3(PEt3)2][(3e), R = H; (3f), R = Et] and [PtW(CO)3(PEt3)2(η5-C2B9H9Me2)](4), formed in a ratio (3e) : (3f) : (4) of ca. 1 : 4 : 1. Moreover, i.r. and n.m.r. studies showed that there was an equilibrium in solution between (3e) and (4). Column chromatography on alumina failed to separate the three complexes completely, and an X-ray study on a crystal from the mixture revealed that co-crystallisation had occurred. The asymmetric unit contains two crystallographically independent and chemically inequivalent molecules. One molecule proved to be (4), having a structure in which the Pt–W bond [2.818(1)A] is bridged by a B–H ⇀ Pt three-centre two-electron linkage. The latter involves the central boron atom in the nido face of the tungsten-ligating η5-C2B9H9Me2 group. The other unit in the crystal exhibited disorder. This arises from its correspondence to the two products (3e) and (3f), which differ only in the occupancy [R = H (20%) or Et (80%)] of the exo-polyhedral site at the central boron in the C2B3nido face of the carbaborane ligand. The structures of (3e) and (3f), with their exo-polyhedral B–Pt σ bonds and tungsten-ligated η5-C2B9H7(R)Me2 groups are thus closely related to that of (3a). Protonation (HBF4·Et2O) of the complexes (3)(L = PMe3, CO, or CNBut) affords the salts [PtW(µ-H)(CO)2(L)(PEt3)2{η5-C2B9H8(CH2C6H4Me-4)Me2}][BF4]. The n.m.r. data (1H, 13C-{1H}, 11 B-{1H}, and 31P-{1H}) for the new compounds are eported, and are discussed in relation to their structures.